Contemporary enterprises depend to great extent on software systems. During the past decades the number of systems has been constantly increasing and these systems have become more integrated with one another. This has lead to a growing complexity in managing software systems and their environment. At the same time business environments today need to progress and change rapidly to keep up with evolving markets. As the business processes change, the systems need to be modified in order to continue supporting the processes. The complexity increase and growing demand for rapid change makes the management of enterprise systems a very important issue. In order to achieve effective and efficient management, it is essential to be able to analyze the system modifiability (i.e. estimate the future change cost). This is addressed in the thesis by employing architectural models. The contribution of this thesis is a method for software system modifiability analysis using enterprise architecture models. The contribution includes an enterprise architecture analysis formalism, a modifiability metamodel (i.e. a modeling language), and a method for creating metamodels. The proposed approach allows IT-decision makers to model and analyze change projects. By doing so, high-quality decision support regarding change project costs is received. This thesis is a composite thesis consisting of five papers and an introduction. Paper A evaluatesa number of analysis formalisms and proposes extended influence diagrams to be employed for enterprise architecture analysis. Paper B presents the first version of the modifiability metamodel. InPaper C, a method for creating enterprise architecture metamodels is proposed. This method aims to be general, i.e. can be employed for other IT-related quality analyses such as interoperability, security, and availability. The paper does however use modifiability as a running case. The second version of the modifiability metamodel for change project cost estimation is fully described in Paper D. Finally, Paper E validates the proposed method and metamodel by surveying 110 experts and studying 21 change projects at four large Nordic companies. The validation indicates that the method and metamodel are useful, contain the right set of elements and provide good estimation capabilities. / QC20100716